The use of rotating polygon scanning mirrors in laser printers to provide a beam sweep or scan of the image of a modulated light source across a photoresistive medium such as a rotating drum is well known. More recently, there have been efforts to use a much less expensive flat mirror with a single reflective surface such as a mirror oscillating in resonance to provide the scanning beam. These scanning mirrors provide excellent performance at a very advantageous cost. However, because the permanent drive magnet is mounted on the resonant mirror itself, the mass balance of the magnet and other rotating elements have very close and critical tolerances. Robust mounting brackets typically have been used to mount the torsional hinge mirrors to a using device to help assure a stable platform. However, distortion of the bracket itself due to mounting stresses and/or different CTE (coefficient of thermal expansion) between a mirror and the bracket can also produce sufficient stress in the mirror bracket that will cause the resonant frequency of the scanning mirror to change beyond acceptable limits or even destroy the mirror.
Texas Instruments presently manufactures mirror MEMS devices fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100 to 115 microns using semiconductor manufacturing processes. The reflective surface of the mirror may have any suitable perimeter shape such as oval, elongated elliptical, rectangular, square or other. Single axis mirrors include the reflective surface portion and a pair of torsional or full hinges, which extend to a support frame or alternately the hinges may extend from the mirror portion itself to a pair of hinge anchors. Other mirror embodiments use a single torsional hinge to eliminate hinge stress.
U.S. Patent Application No. 2004/0027449 describes various techniques for creating the pivotal resonance of the mirror device about the torsional hinges. Thus, by designing the mirror hinges to resonate at a selected frequency, a scanning engine can be produced that provides a scanning beam sweep with only a small amount of energy required to maintain resonance.
In addition, the mass balance of the mirror further complicates the task of mounting the resonant frequency within acceptable tolerances. Magnetic drive mechanisms for the mirror typically depend on a permanent magnet mounted to the resonating mirror surface interacting with a drive coil located very close to the mirror. The critical mass balance of the mirror requires that the permanent magnet be designed with a size, thickness and mass having very close tolerances.
Therefore, since applications that use a pattern of light beam scans, such as laser printers and imaging projectors, as well as other uses, require a stable precise drive to maintain a constant scan velocity, the changes in the resonant frequency and scan velocity of a pivotally oscillating device due to temperature variations, careless placement of the magnet on the mirror or out of tolerance size and/or mass, can restrict or even preclude the use of the device in laser printers.
Therefore, it would be advantageous to provide an inexpensive and easily manufactured mirror package that does not require extremely close tolerance as to mounting position.